This invention relates generally to elevators, and more specifically to a hydraulic drive for operating an elevator and disposed inside the elevator shaft on one side of the load-lifting member of the elevator.
Conventional hydraulic drive means in an elevator shaft for a load-lifting member that is to be lifted to a considerable height comprise a plurality of hydraulic ram cylinders so disposed that the rams become operative in succession. The lengths of the piston rods of the ram cylinders must be so designed that there is no danger of the piston rods bucking under the load. This danger is present in forms of construction in which the ram cylinders are designed jointly to lift the load-lifting member to its maximum height. Frequently this will not be sufficient to meet particular needs. Arrangements in which the hydraulic rams operate in succession are naturally expensive because of the need for the provision of a plurality of such rams, which are costly. Moreover, such drives are of complicated construction. Furthermore, in the event of their being disposed on the side of the load-lifting member, e.g. of an elevator cage, they call for the provision of complicated coupling means for automatically coupling and uncoupling the rams to and from the cage. A hydraulic drive for operating an elevator has already been proposed which comprises a hydraulic cylinder containing two ram pistons connected by an interposed hollow cylindrical member having an external diameter corresponding with the internal diameter of the hydraulic cylinder. This cylindrical member is fitted with a carrier resembling a bracket arm which projects to the outside through an axial slot in the wall of the hydraulic cylinder for carrying the elevator cage. The slot is sealed by a sealing strand which partly embraces the hydraulic pistons and extends through the cylindrical member to permit the carrier arm on the cylindrical member to project through the slot in the hydraulic cylinder. However, in the event of the cage being required to ascend to a major height a pressure-tight seal of the slot in the wall of the hydraulic cylinder is impossible to achieve because of the magnitude of the pressure range, particularly bearing in mind that continuous operation leads to premature wear of the seal.